Santorini, officially Thira, is an island located in the southern Aegean Sea, about 200 km (120 mi) southeast from Greece's mainland.

Santorini is essentially what remains after an enormous volcanic explosion that destroyed the earliest settlements on what was formerly a single island, and created the current geological caldera. A giant central lagoon, more or less rectangular, and measuring about 12 by 7 km (7.5 by 4.3 mi). The main island slopes downward from the cliffs to the surrounding Aegean Sea.

It is the most active volcanic centre in the South Aegean Volcanic Arc, though what remains today is chiefly a water-filled caldera. The region first became volcanically active around 3–4 million years ago, though volcanism on Thera began around 2 million years ago with the extrusion of dacitic lavas from vents around the region of Akrotiri.

The island is the site of one of the largest volcanic eruptions in recorded history: the Minoan eruption (sometimes called the Thera eruption), which occurred some 3600 years ago at the height of the Minoan civilization. The eruption left a large caldera surrounded by volcanic ash deposits hundreds of feet deep and may have led indirectly to the collapse of the Minoan civilization on the island of Crete, 110 km (68 mi) to the south, through a gigantic tsunami. This theory is not, however, supported by chronology, in that the collapse of the Minoan civilization did not occur at the date of the tsunami, but some 90 years later. Another popular theory holds that the Thera eruption is the source of the legend of Atlantis.